We
said earlier that it is important that you understand some of the ways in which
pressure affects liquids and gases and some of the relation-ships between
pressure, temperature, and volume in gases.

The
boiling point of any liquid varies according to the pressure on the liquid-the
higher the pressure, the higher the boiling point. You should remember that
condensing a gas to a liquid is just the reverse process of boiling a liquid
until it vaporizes. The same pressure and temperature relationship is required
to produce either change of state.

Water
boils at 212°F at the atmospheric pressure of 14.7 psia, at 80°F under a vacuum
of 29 inches of mercury, and at 489°F at a pressure of 600 psig. Refrigerants
used in vapor com-pressor cycle equipment usually have much lower boiling
points than water under any given pressure. However, these boiling points also
vary according to pressure. At atmospheric pressure, for example, the
refrigerant R-12 boils at - 21.6°F. At 30 psig, R-12 boils at 32°F, which is
the freezing point of water. You should see that R-12 cannot exist as a liquid
at ordinary temperatures. It must be confined within a container or closed
space.

When
the temperature of a liquid is raised to the boiling point corresponding to its
pressure and if application of heat is continued, the liquid will begin to boil
and vaporize. The vapor that is formed will remain at the same temperature as
the boiling liquid as long as it is in contact with the liquid. A vapor CANNOT
be superheated as long as it is in contact with the liquid from which it is
being generated.

The
pressure-temperature-volume relation-ships of gases are expressed by Boyle’s
law, Charles’s law, and the general gas law or equation. We will briefly
discuss each of these laws.

BOYLE’S
LAW states that the volume of any dry gas varies inversely with its absolute
pressure, provided the temperature remains constant. This law may also be
expressed as the formula wherein V1 is the
original volume of the gas, P1 is its
original absolute pressure, V2 is its new
volume, and P2 is its new absolute
pressure.

CHARLES’S
LAW states that the volume of a gas is directly proportional to its absolute
temperature, provided the pressure is kept constant. The equation for Charles’s
law is

The
GENERAL GAS EQUATION combines Boyle’s law and Charles’s law. It expresses the
interrelationship of the volume, the absolute pressure, and the absolute
temperature of gases. The general gas law is expressed by the formula

In
Boyle’s law, Charles’s law, and the general gas law, the equations indicate the
nature of the interrelationship of the pressure, the volume, and the
temperature of any gas. You probably will not find it necessary to use the
equations themselves, but you should have a thorough understanding of the
principles they express. Let’s summarize them:

1.
When TEMPERATURE is held constant, increasing the pressure on a gas causes a
proportional decrease in volume. Decreasing the pressure causes a proportional
increase in volume.

2.
When PRESSURE is held constant, increasing the temperature of a gas causes a
proportional increase in volume. Decreasing the temperature causes a
proportional decrease in volume.

3.
When the VOLUME is held constant, increasing the temperature of a gas causes a
proportional increase in pressure. Decreasing the temperature causes a
proportional decrease in pressure.

In
discussing the effects of pressure on a gas, we have pointed out that the
volume and the temperature of gas are different AFTER the pressure has been
changed. It is important to note, however, that a temperature change normally
occurs in a gas WHILE the pressure is being changed. Compressing a gas raises
its temperature; allowing a gas to expand lowers its temperature. As you will
see, these two facts are important to your understanding of the operating
principles of the refrigeration cycle.